The present invention concerns a friction clutch for a thermal engine motor vehicle, and relates particularly to a clutch equipped with a device for taking up the play due mainly to the wear on the friction linings, the said device operating continuously as the wear occurs on the said linings.
A conventional friction clutch generally comprises a reaction plate, possibly in two parts in order to form a damping flywheel, fixed rotationally to a first shaft, usually a driving shaft such as the crankshaft of the internal combustion engine, and supporting through its external periphery a cover to which at least one pressure plate is attached.
The pressure plate is rotationally fixed to the cover and to the reaction plate whilst being able to move axially under the action of the controlled axially acting resilient means, generally a metallic diaphragm bearing on the cover, whilst a friction disc, carrying friction linings at its external periphery, rotationally fixed to a shaft, usually a driven shaft such as the input shaft of the gearbox, is interposed between the pressure plate and the reaction plate so as to be clamped between them when the clutch is in the engagement position. The diaphragm controls the axial movement of the pressure plate when it is actuated by a clutch release bearing.
During the service life of such a clutch, the friction linings and, to a lesser extent, the counter materials, pressure plate and reaction plate, wear, which causes a variation in the clamping force between the friction disc on the one hand and the pressure and reaction plates on the other hand, because of the changes in the working conditions of the diaphragm. The clutch travel also varies, as well as the force necessary for declutching. By providing such a clutch with a device for taking up the play due mainly to the wear on the linings, these drawbacks are avoided.
Through the document FR 2 426 834, a play take-up device is known in which a first plate, in this case the pressure plate, is in two coaxial parts able to be moved axially with respect to one another so that the overall thickness of the first plate increases as the wear increases.
In this document, the two coaxial parts of the first plate are able to be moved axially with respect to one another by virtue of a take-up device which comprises ramp means carried in a complementary fashion by the two coaxial parts, which are also able to be moved in rotation with respect to one another in the direction of the increase in the said overall thickness and prevented from turning with respect to one another in the opposite direction to the previous one by a unidirectional means. The relative rotation movement of the coaxial parts is controlled by a trigger device sensitive to the state of wear and implemented by a circumferentially acting resilient means coupled between the said two coaxial parts.
Such a device effectively continuously takes up the play due to the wear on the friction linings. There has also been proposed, in particular in the document FR-A-2 750 467, a clutch of the above type which is of more simple design, having a lesser number of parts.
According to this document, a friction clutch, in particular for a motor vehicle, of the type comprising a reaction plate intended to be rotationally fixed to a driving shaft, a friction disc, carrying friction linings at its external periphery, intended to be rotationally fixed to a driven shaft, a pressure plate, a cover fixed to the reaction plate, axially acting resilient means acting between the cover and the pressure plate, the pressure plate being rotationally fixed to the cover whilst being able to move axially with respect to it, one of the two plates, the reaction plate or the pressure plate, referred to as the first plate, consisting of two half-plates, a so-called external half-plate and a so-called internal half-plate, the internal half-plate being the one which faces a friction lining and which is intended to cooperate with it, the two half-plates being rotationally fixed to the driving shaft but able to be moved axially, with respect to each other, under the action of a device for taking up the play due to the wear mainly on the friction linings, which wear take-up device comprises at least one pair of circumferentially disposed complementary ramps associated respectively with each of the two half-plates, one of the ramps, referred to as the support ramp, being rotationally fixed to one of the half-plates, the other ramp, referred to as the adjustment ramp, being fixed to the other half-plate and adapted to move circumferentially with respect to the support ramp, in the direction of increase in the overall thickness of the said first plate, referred to as the take-up direction, and prevented from moving in the other direction by a so-called unidirectional means, the said circumferential movement being controlled by a release sensitive to the state of wear.
According to this document, the adjustment ramp is provided at the periphery of an inertia piece, so that the lack of periodicity of the thermal engine of the motor vehicle is taken advantage of in order to make the complementary ramps effect their relative circumferential movement in the direction of taking up of play. This is because, as is known, lack of periodicity in rotation results in alternating angular accelerations which may achieve more or less two thousand radians per second squared; by virtue of the inertia piece subjected to these angular accelerations, when the clutch is disengaged and consequently the two half-plates are no longer under the axial force of the axial acting resilient means, the adjustment ramp turns in the taking-up direction with respect to the support ramp, since the unidirectional means prevent it from turning in the other direction. Thus it is not necessary to provide a circumferentially acting resilient means between the two so-called coaxial parts and the lack of periodicity is taken advantage of.
As can also be seen, the functioning of the play take-up device is automatic. Experience has shown that cases may occur where this automatic functioning is a drawback since it leads to a take-up operation which is not related to the wear, an operation known as over-takeup.
This is the case for example at very low speed, below the tickover speed, and in particular when the reaction plate, or flywheel, is a double-damping flywheel where the natural frequency of the vibrations is situated in this range of low speeds and produces vibration causing the external half-plate to move and take up a xe2x80x9cplayxe2x80x9d which does not exist.
Other parasitic phenomena may occur at higher speeds, for example above two thousand revolutions per minute.
The aim of the present invention is to avoid these drawbacks.
According to the invention, a friction clutch for a motor vehicle with a thermal engine, of the type having a reaction plate intended to be rotationally fixed to a driving shaft, a friction disc, carrying friction linings at its external periphery, intended to be rotationally fixed to a driven shaft, a pressure plate, a cover fixed to the reaction plate, axially acting resilient means acting between the cover and the pressure plate, the pressure plate being rotationally fixed to the cover whilst being able to move axially with respect to it, one of the pieces, the reaction plate or the pressure plate or cover, consisting of two half-pieces, able to be moved axially, with respect to one another, under the action of a device for taking up the play due to wear, which play take-up device comprises at least one pair of circumferentially disposed complementary ramps axially associated respectively with each of the two half-pieces, one of the ramps, being adapted to move circumferentially with respect to the other ramp in the direction of an increase in the overall thickness of the said piece, referred to as the take-up direction, and prevented from moving in the other direction by a so-called unidirectional means, the said circumferential movement being controlled by a release sensitive to the state of wear, one of the ramps being provided at the periphery of an inertia piece, so that the lack of periodicity of the thermal engine of the motor vehicle is taken advantage of in order to make the complementary ramps make their relative circumferential movement in the direction of taking up the play, is characterised by the fact that it comprises control means adapted to control the play take-up device by allowing its functioning or not.
Advantageously, the said piece is one of the two plates, the reaction plate or the pressure plate, referred to as the first plate, consisting of two half-plates, a so-called external half-plate and a so-called internal half-plate, the internal half-plate being the one which faces a friction lining and which is intended to cooperate with it, the two half-plates being rotationally fixed to the driving shaft but able to be moved axially, with respect to one another, under the action of the play take-up device, one of the ramps, known as the support ramp, being rotationally fixed to one of the half-plates, the other ramp, known as the adjustment ramp, being adapted to move circumferentially with respect to the support ramp in the direction of an increase in the overall thickness of the said first plate, the adjustment ramp being provided at the periphery of the inertia piece.
The support ramp is preferably carried by the internal half-plate.
Advantageously, the adjustment ramp is carried by the external half-plate.
According to a preferred embodiment, the first plate is the pressure plate.
Advantageously, the axially acting resilient means consist of a diaphragm, and the external half-plate is provided with a support bead for the diaphragm.
The adjustment ramp is preferably carried by the internal periphery of the external half-plate; in a variant, the adjustment ramp is carried by the external periphery of the external half-plate.
The adjustment and support ramps are preferably helical threads whose axis is the axis of the clutch.
In a variant, the adjustment ramp consists of a series of ramps extending circumferentially at the end of an annular piece made from cropped and bent sheet metal.
The unidirectional means is preferably a split segment rotationally fixed to the first plate at at least one point.
In a variant, the unidirectional means is a helically wound snap ring rotationally fixed to the first plate at at least one point.
The control means are preferably controlled by the axially acting resilient means.
Advantageously, the control means comprise a break in the form of a tongue, secured or not to the axially acting resilient means, adapted to cooperate with one of the ramps; the tongue is in the form of a pin mounted for articulation.
According to another embodiment, the control means are sensitive to the centrifugal force and/or to an acceleration force.
The control means preferably consist of a U-shaped resilient tongue carried by the cover.
In a variant, the control means comprise a lever articulated on the internal half-plate and its two arms are adapted to cooperate with the unidirectional means.
According to another variant, the control means comprise one (or more) cams mounted so as to be eccentrically articulated on the internal half-plate and adapted to cooperate with the external half-plate.
The control means comprise a snap ring carried by the internal half-plate and adapted to cooperate, under the effect of the centrifugal force, with the external half-plate in order to prevent the latter from turning in the direction permitted by the unidirectional means.
The unidirectional means is a snap ring and the control snap ring and the unidirectional snap ring are in a single piece.
The unidirectional means is a snap ring and a weight, sensitive to the centrifugal force and/or to the angular acceleration, is associated with it in order to brake, beyond a certain rotation speed or a certain angular acceleration, the external half-plate.
Preferably, at least one of the snap rings is subjected to the circumferential action of a spring.
According to yet another variant, the control means comprise an elastic means exerting a substantially tangential snap ring and adapted to cooperate with the external half-plate under the action of the unidirectional means moved by inertia.
The adjustment ramps cooperate with counter-ramps or support ramps carried by a ring fixed to the internal half-plate by friction, which is elastically controlled.
According to another embodiment, the piece consisting of two half-pieces is the cover.
The internal half-cover preferably consists of the central part of the bottom of the cover, being externally threaded, whilst the external half-cover is internally threaded so that the internal and external half covers are screwed to onexe2x80x94another.
Advantageously, a brake in the form of a wedge, controlled by the diaphragm, can be inserted between the two half-covers.